by Jim Baggott
On the surface, this is yet another take on the multiverse idea. But the brane multiverse is rather different. The many worlds interpretation and the inflationary multiverse leave us with a rather static picture. The parallel universes are almost by definition detached one from another and they do not interact. But in M-theory, branes are dynamic objects. They move around (in the bulk). And they can interact.
What happens when brane universes collide? They don’t just pass through each other like ghostly ships in the night. M-theory suggests that a collision between braneworlds could be very violent.
In 2001, Princeton theorists Paul Steinhardt and Justin Khoury, working with Burt Ovrut at the University of Pennsylvania and Neil Turok, then at Cambridge University, used heterotic M-theory to study the effects of colliding braneworlds. They concluded that, under the right circumstances, the collision of a ‘visible’ 3-brane containing four large spacetime dimensions and standard model particles with an ‘invisible’ 3-brane moving slowly through the bulk could in principle trigger a cataclysmic release of energy. A small proportion of the kinetic energy (the energy of motion) of the invisible brane is converted into hot radiation, which bathes the matter in the visible brane.
The hot radiation is later recognized as a hot big bang. It accelerates the expansion of spacetime without requiring the mechanism of inflation as used in the ACDM model and eternal inflation. The collision resets the visible brane’s time clock, and, to all intents and purposes, the universe we observe today is born. The theorists wrote:
Instead of starting from a cosmic singularity with infinite temperature, as in conventional big bang cosmology, the hot, expanding universe in our scenario starts its cosmic evolution at a finite temperature. We refer to our proposal as the ‘ekpyrotic universe’, a term drawn from the Stoic model of cosmic evolution in which the universe is consumed by fire at regular intervals and reconstituted out of this fire, a conflagration called ekpyrosis. Here, the universe as we know it is made (and, perhaps, has been remade) through a conflagration ignited by collisions between branes along a hidden fifth dimension.18
As far as I can tell, the term ‘ekpyrotic universe’ didn’t catch on. Some have referred to the scenario instead as the big splat, which is something of a misnomer, as the branes don’t so much splat as bounce off each other.
I should make clear that Steinhardt and Turok and their colleagues do not promote their work as an argument in favour of a brane multiverse. Rather, they have developed a cosmology which seeks to explain our universe in terms of an eternal cycle: two branes collide and bounce apart, but never separate far enough to escape their mutual gravitational influence (which is manifested in our universe as dark energy). Some trillions of years later the branes collide again, the visible universe is ‘reset’ and the cycle repeats.
In this colliding braneworld scenario, the pattern of temperature variations observed in the CMB radiation is the result of quantum fluctuations in both braneworlds at the point of collision. The branes are not perfectly ‘flat’. Quantum uncertainty means that the braneworlds come into contact first in places where the amplitudes of fluctuations in the direction of the bulk dimension are high. These cause ‘hotspots’ — places in the visible brane where the radiation temperature is higher than the average.
Steinhardt, Turok and their colleagues have continued to develop this cyclic cosmology, which they now refer to as a ‘phoenix universe’, championing it as a viable alternative to cosmologies based on inflation. In 2008, Steinhardt and Princeton theorist Jean-Luc Lehners adapted the model to prevent spacetime collapsing into an amalgam of black holes. In this revised model, much of the universe is destroyed but a small volume (of the order of a cubic centimetre) survives the ekpyrotic phase (hence the term ‘phoenix’). ‘As tiny as a cubic centimetre may seem, it is enough to produce a flat, smooth region a cycle from now at least as large as the region we currently observe. In this way, dark energy, the big crunch and the big bang all work together so that the phoenix forever arises from the ashes, crunch after crunch after crunch.’19
The reality check (II)
There are other kinds of multiverse theory that unfortunately we don’t have the space to consider here. In The Hidden Reality, Greene identifies nine different types, including many worlds, the inflationary multiverse, the brane and cyclic multiverses and the cosmic landscape.
This is surely an extraordinary situation. At great risk of repeating myself, I want to summarize briefly the steps that have brought us here.
The current authorized version of reality consists of a collection of partially connected theoretical structures — quantum theory, the standard model of particle physics, the special and general theories of relativity and the ΛCDM model of big bang cosmology. There remain many significant problems with these structures and we know this can’t be the final story. But there is little guidance available from experiment and observation. There are no big signs pointing us towards possible solutions.
Into this vacuum a number of new theoretical structures have been introduced. They offer potential solutions for some of the problems but they are inevitably motivated by the theorists’ instincts and their mathematical and personal preferences. The theories are constructed on a network of assumptions that we are obliged to accept at face value.
So, superstring theory is founded on the assumption that elementary particles can be represented by vibrations in one-dimensional filaments of energy. To this we add the assumption that there exists a fundamental spacetime symmetry between fermions and bosons. As the theory then demands a total of nine spatial dimensions, we assume that six dimensions are compactified into a Calabi—Yau space. There is no experimental or observational evidence for any of these assumptions.
In a separate series of developments, the quantum measurement problem is resolved by assuming that all the different possible outcomes of a measurement are realized in different equally real worlds which either split from one another or exist in parallel. Eternal inflation is a characteristic of certain cosmological models. These models describe a multiverse consisting of bubbles of inflating spacetime triggered by quantum fluctuations in a vast inflaton field. We assume our universe is a relatively unexceptional bubble in this multiverse. We further assume that each of these universes can be characterized by one of the 10500 different possible ways of compactifying the six extra spatial dimensions demanded by superstring theory. In an alternative scenario, the hot big bang origin of our own universe could be the result of a collision between two braneworlds. There is no experimental or observational evidence for any of these assumptions.
Let’s just check to see if we’ve understood this correctly. We live in a multiverse, ‘surrounded’ by parallel universes that by definition we cannot experience directly. We can never verify the existence of these universes, and must look instead for evidence that betrays their existence indirectly in the physics of our own universe.
Of course, there is no evidence in the physics of the authorized version of reality, so we must look to the physics of superstrings or M-theory. And look! The fact that there is no preferred choice of Calabi—Yau shape from the 10500 different possibilities is taken to imply that our universe is far from unique. There must be many, many other kinds of universe. Quod erat demonstrandum.
Justifying a multiverse theory because it is implied by superstring theory might just be acceptable if superstring theory was an already accepted description of the universe we experience. But it’s not. In fact, one of the things standing in the way of superstring theory’s acceptance is its inability to predict anything about our own universe. And this is partly because, with 10500 different Calabi—Yau spaces to choose from, virtually anything and everything is possible. There’s nothing in the theory that allows us to pick out the space that describes our universe uniquely.
The multiverse theory is justified by superstring theory but superstring theory cannot be proved because we live in a multiverse.
We’ve not j
ust crossed a line here. We’re so far over it that, to quote Matt LeBlanc as Joey in an episode of Friends, the line is a dot to us. To be fair, some of the most recent popular presentations of multiverse theories have included sections titled ‘Is this science?’ or something similar. Some authors argue that the multiverse theory might be testable through the observation of legacy effects in the CMB from a big bang triggered by colliding branes. Others argue that the multiverse must be science because the cosmic landscape is derived from superstring theory and superstring theory is science. Frankly, these look very much like attempts to clutch at straws, and none are convincing.
Reference to the six Principles described in Chapter 1 would lead us to conclude that theories constructed on many worlds and the multiverse are not testable in principle. To those who would argue that there are genuine instances in which parallel universes could impress themselves on the physics we observe in our universe, I would respectfully suggest that physicists will struggle to demonstrate that any such evidence is free from ambiguity. I honestly can’t believe that it will be beyond the wit of theorists to find alternative physical explanations for such instances based on theories requiring one — and only one — universe.
Of course, this is just an opinion.
Dealing with the fallout
As you may have gathered by now, I’m rather exasperated by the relentless pursuit of SUSY, superstring theory and M-theory, and the widening gap between increasingly implausible theoretical speculations and the practicalities of experiment and observation. But I also believe that the willingness to embrace multiverse theories — what Peter Woit refers to as ‘multiverse mania’ — is doing serious damage to the cause of science itself.*
Firstly, there is mis-selling of the kind I highlighted for the superstring programme. Multiverse theories have divided the physics community and the debate is growing in vocal intensity. Although there are many notable advocates, such as Susskind, Tegmark, Greene (a relatively recent convert) and Martin Rees (Britain’s Astronomer Royal), there are also notable dissidents. These include David Gross, Steinhardt and Lee Smolin. Despite this disagreement, the multiverse is still paraded in the popular science media as an accepted body of scientific theory.
Thus, in an ‘ultimate guide’ to the multiverse published in the respected UK popular science weekly New Scientist in November 2011, science writer Robert Adler opens with:
Whether we are searching the cosmos or probing the subatomic realm, our most successful theories lead to the inescapable conclusion that our universe is just a speck in a vast ocean of universes.
He does not explain how ‘success’ is defined. Nowhere in this article can I find any reference to the fact that this is not a universally accepted theory, or that multiverse theories are not actually ‘inescapable’. Or that a debate is raging about whether this is even science. On the contrary, he continues:
Three decades after the concept was born, many researchers now agree that at least some kinds of multiverse stand on firm theoretical foundations, yield predictions that astronomers and particle physicists can test, and help explain why our universe is the way it is.20
But how many is ‘many’? And, should you be in any doubt, please be reassured that no multiverse theory of any kind can explain why our universe is the way it is.
The second area of concern relates to the really rather obvious fact that multiverse theories represent an enormous cop-out. By arguing that all the different measurement outcomes and all the different possible universes are realized in the multiverse, theorists duck the challenge to understand why we experience these outcomes in our universe and why our universe has the physical constants, laws and spectrum of particles that it has. Greene himself acknowledges this problem in The Hidden Reality:
By invoking a multiverse, science could weaken the impetus to clarify particular mysteries, even though some of those mysteries might be ripe for standard, nonmultiverse explanations. When all that was really called for was harder work and deeper thinking, we might instead fail to resist the lure of multiverse temptation and prematurely abandon conventional approaches.21
The multiverse answer is that all possible universes exist in parallel and we just happen to find ourselves in one that supports our form of life.* Of course, this is no answer at all. Greene goes on to defend the multiverse approach, arguing that to abandon it because it could be a blind alley is equally dangerous.
Finally, and most importantly, we must be concerned about the implications of multiverse theories for the future development of science itself. The multiverse theorists know that they are on weak ground regarding the Testability Principle, and rather than admit that their theories are not science, they argue instead that the rules of science must be adapted to accommodate this kind of metaphysical speculation.
They want to change the very definition of science. This is a very slippery slope.
* This does not necessarily contradict the observation of the kinds of macroscopic superpositions described in Chapter 3, in which a wavefunction is formed in which billions of electrons flow in opposite directions around a superconducting ring. It just means that we have to work very hard to stop it from decohering.
* Strictly speaking, the term ‘universe’ means ‘everything there is’, but it quickly gets confusing if we refer to lots of parallel universes as ‘the universe’. The logic that prevails here is that we consider our known universe of visible stars, galaxies, dark matter and dark energy as merely one among many such universes that exist alongside ours in parallel. We call this collection of different parallel universes the ‘multiverse’.
* Strictly speaking, the probabilities are given by |0.866|2 and |0.500|2. We use the modulus-squares because the coefficients could be complex (they could include i, the square root of-1).
* See www.math.Columbia.edu/~woit/wordpress/?cat=10.
* We will further explore this kind of reasoning in Chapter 11.
10
Source Code of the Cosmos
Quantum Information, Black Holes and the Holographic Principle
The physicist has to limit himself very severely: he must content himself with describing the most simple events that can be brought within the domain of our experience; all events of a more complex order are beyond the power of the human intellect to reconstruct with the subtle accuracy and logical perfection the theoretical physicist demands.
Albert Einstein1
The Reality Principle introduced in Chapter 1 argues that reality consists of things-in-themselves which are, by definition, inaccessible to us and of which we can never hope to gain knowledge. Science is the process by which we continue to refine our knowledge of the things-as-they-appear or the things-as-they-are-measured, from which we can infer what reality is ‘really’ like if we first assume that reality has objective, independent existence.
As we have no way of knowing what such an independent reality is actually like, this leaves us with some considerable freedom to speculate.
The reductionists among us might be tempted simply to conclude that reality must be ultimately composed of irreducible bits or fields of ‘stuff’, however we choose to define these. These bits or fields impress themselves on our empirical reality of observation and measurement in ways that are hopefully logical and accessible to human reason. Whatever reality is, this stuff manifests or projects itself to produce effects which we interpret as quarks and leptons and force particles, and so on.
But what if, instead, there is no ‘stuff’?* After all, Einstein set some remarkable precedents in the early twentieth century. In the special and general theories of relativity he showed that space and time are not absolute, they are relative. They depend on the things that exist within them and, it can be argued, they have no existence independently of these things.
The Higgs mechanism turns mass into a secondary quality, like colour. We begin to get the sense that, though tangible, mass is an effect — it is the result of things interacting with the Higgs field �
� rather than something primary, something that exists independently of such interactions.
And then the experimental violation of Leggett’s inequality tells us that what we call ‘properties’ — for example, the spin orientations of quarks and leptons — cannot be considered to be inherently pre-existing attributes of these things. They are instead products of the process of measurement — they are the ‘projections’ (if that’s the right word) of the things-in-themselves into our empirical reality of measurement.
The story of modern physics is really the story of the not-so-gradual undermining of our naïve ideas about reality. It tells of the erosion of our broadly common-sense, taken-for-granted notions and their replacement by uncertainty and doubt. Can we expect this erosion of confidence to continue indefinitely?
What if we stripped away all the things we call ‘properties’, the things that define what it means for a quantum particle to be present in a specific quantum state, the things that determine how the particle is going to behave. What would be left?
There are at least two possible answers to this question. The first is that by stripping away all the empirical dressing — energy, mass, spin, space, time, etc. — what we are left with is abstract mathematics. A second answer is that we could consider the irreducible ‘stuff’ of the universe (or multiverse) to be information.
The Mathematical Universe Hypothesis
The principal exponent of the notion that the core description of everything in the universe might be essentially mathematical is MIT theorist Max Tegmark. Now, theorists have always tended to wax rather lyrical about what Eugene Wigner called the ‘unreasonable effectiveness of mathematics in the natural sciences’.2 The creative processes involved in fashioning theories out of the often arcane and esoteric concepts, rules and language of mathematics are arguably equivalent in many ways to art. And just as great art seems to tell us vivid truths about human existence, so perhaps great maths relates deep truths of physical existence.